Recently, in order to improve the throughput for the next generation WiFi systems, orthogonal frequency division multiplexing access (OFDMA) has been actively discussed as a key feature by the standardization body (e.g., IEEE 802.11 ax). The major virtues of OFDMA are 1) the possibility of reducing the channel contention time by fine frequency domain resource utilization and 2) the possibility of exploiting frequency and multiuser diversity via frequency selective scheduling.